This invention relates to medical devices and, in particular, a multi-electrode catheter and techniques therefor of employing multi-phase radio-frequency power source for ablation of endocardiac tissues.
Cardiac dysrhythmias are commonly known as irregular heart beats or racing heart. Two such heart rhythm irregularities are the Wolff-Parkinson-White syndrome and atrioventricular (AV) nodal reentrant tachycardia. These conditions are caused by an extraneous strand of muscle fiber in the heart that provides an abnormal short-circuit pathway for electric impulses normally existing in the heart. For example, in one type of the Wolff-Parkinson-White syndrome the accessory pathway causes the electric impulses that normally travel from the upper to the lower chamber of the heart to be fed back to the upper chamber. Another common type of cardiac dysrhythmias is ventricular tachycardia (VT), which is a complication of a heart attack or reduction of blood supply to an area of heart muscle, and is a life threatening arrhythmia.
In the treatment of cardiac dysrhythmias, non-surgical procedures such as management with drugs are favored. However, some dysrhythmias of the heart are not treatable with drugs. These patients are then treated with either surgical resection of VT site of origin or by Automatic implantable cardiovertor defibrillator (AICD). Both procedures have increased morbidity and mortality and are extremely expensive. Even AICD needs major surgical intervention. In addition, some patients of advanced age or illness cannot tolerate invasive surgery to excise tachycardia focus which causes dysrhythmias.
Techniques have been developed to locate regions of tachycardia and to disable their short-circuit function. Electrical energy shocks are applied to ablate the cardiac tissues in those regions so as to produce scars and interrupt conduction.
The regions to be ablated are usually determined by endocardiac mapping. It is a technique that typically involves percutaneously introducing an electrode catheter into the patient. The electrode catheter is passed through a blood vessel, like femoral vein or aorta and thence into an endocardiac site such as the atrium or ventricle of the heart. A tachycardia is induced and a continuous, simultaneous recording made with a multichannel recorder while the electrode catheter is moved to different endocardiac positions. When a tachycardial focus is located as indicated in an electrocardiogram recording, it is marked by means of a fluoroscope image.
Upon locating of the tachycardial focus, ablation of cardiac arrhythmias is typically performed by means of a standard electrode catheter. The electrical energy shocks is used to create a lesion in the endocardiac tissues adjacent (i.e. underneath) the standard electrode catheter. By creating one or more lesions, the tachycardial focus may be turned into a region of necrotic tissue, thereby disabling any malfunctions.
Conventional catheter ablation techniques have typically employed a catheter with a single electrode at its tip as one electrical pole. The other electrical pole is formed by a backplate in contact with a patient's external body part. These techniques have been used successfully for interruption or modification of conduction across the atrioventricular (AV) junction in AV nodal reentrant tachycardia; for interruption of accessory pathway in patients with reentrant tachycardia due to Wolff-Parkinson-White Syndrome; and for ablation in some patients with ventricular tachycardia.
In one technique, high voltage direct current (DC) in the range of 100-300 joules is applied across the electrode and the backplate to effect ablation. Direct current energy source using the standard electrode catheter can produce a lesion size larger than the footprint of the electrode. However, the lesion dimensions are variable at the same energy output and they do not have clear demarcation from the surrounding tissues. Additionally, high voltage techniques have other undesirable side-effects such as barotrauma and the lesions formed could become proarrhythmic.
Another technique is to apply a radio-frequency (RF) source to a standard electrode catheter. The RF source is typically in the 600 kHz region and produces a sinusoidal voltage between two wires. When this is delivered between the distal tip of a standard electrode catheter and a backplate, it produces a localized RF heating effect. It causes a well defined, discrete lesion slightly larger than the tip electrode. This simple RF ablation technique creates lesion size sufficient for interruption of AV junction or accessory pathway.
RF ablation is preferable to DC ablation because it does not need anesthesia and produces more circumscribed and discrete lesions and avoids injury caused by high voltages as in DC shock. Generally, catheter ablations of AV junction using standard electrode catheter with DC or RF energy for treating drug resistant supraventricular tachycardia have high success rate with very low incidence of complications.
However, in ventricular tachycardia (VT), endocardiac mapping with a standard electrode catheter can locate the exit site of ventricular tachycardia to within 4-8 cm.sup.2 of the earliest site recorded by the catheter. A standard electrode catheter typically has a maximum electrode tip area of about 0.3 mm.sup.2. Therefore, the lesion created by the simple RF technique delivered through a standard electrode catheter may not be large enough to ablate the ventricular tachycardia. Attempts to increase the size of lesion by regulation of power and duration by increasing the size of electrode or by regulating the temperature of tip electrode have met with partial success.
In order to increase the size of the lesion, an orthogonal electrode catheter array (OECA) with four peripheral electrodes and one central electrode has been proposed. Such an OECA has been disclosed by Dr. Jawahar Desai in U.S. Pat. No. 4,940,064, issued Jul. 10, 1990, for use in both mapping and ablation of endocardiac sites.
The four peripheral electrodes are actuable from a retracted or collapsed mode. When fanned out, the four peripheral electrodes and the central electrode form an electrode array that typically covers an area of about 0.8 cm.sup.2. When used with a conventional RF power source in conjunction with a backplate, the five connecting electrodes will typically produce five lesion spots distributed over the area spanned by the electrode array. However, this arrangement has been found to be unsatisfactory as there are substantial areas between the electrodes that remain unablated. Increasing the power only results in charring of the tissues and early fouling of the electrodes by coagulum formation.
Thus, it is desirable, especially for treating ventricular tachycardia, to have catheter ablations that produce substantially larger, deeper and more uniform lesions than those produced by conventional RF schemes described above.